Universal modulator

ABSTRACT

A modulator generates a combined signal consisting of audio and video signals, and converts the combined signal to one of a plurality of frequencies in dependence upon a desired output frequency and broadcast standard. The modulator includes a summing amplifier, a first frequency synthesizer and a second frequency synthesizer. The summing amplifier has a first input for receiving a video signal, a second input for receiving a first audio signal, a third input for receiving a second audio signal, and an output for outputting a modulated summed signal. The first frequency synthesizer generates a first frequency for mixing with the modulated summed signal to generate a high intermediate frequency (HI-IF) signal. The second frequency synthesizer generates a second frequency for mixing with the HI-IF signal to generate a desired RF output signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/857,010 filed May 29, 2001, which claims the benefit of U.S.provisional patent application 60/110,254 filed on Nov. 30, 1998, whichare incorporated by reference as if fully set forth.

BACKGROUND

The present invention generally relates to cable television (CATV) andconsumer video communication systems. More particularly, the inventionrelates to a dual-conversion universal modulator having programmablesynthesized phase-locked loop oscillators driving their respectivemixers which select a specific HI-IF frequency depending upon whatoutput frequencies or standards are desired. Such standards includeNTSC, PAL, NICAM, DIN, SECAM and any other known standard.

To allow reception of more than the 12 VHF channels on an oldertelevision receiver, most CATV systems require a settop terminal at asubscriber's location. Today, settop terminals not only provide a meansfor accepting a plurality of channels broadcast with varying bandwidthsand guardbands for forward and reverse frequencies, but they also securepay television services from unauthorized viewing. Other functionsinclude decoding digital video and audio, interactive services, creatingpersonalized viewer channels and the like.

In addition to the conversion from a cable transmission to a standardoutput frequency, a variety of descrambling techniques are employeddepending upon the techniques used at a system headend. CATV equipmentmanufacturers are developing more sophisticated scrambling techniquesusing complicated encryption methods and digital processing to thwartpirating.

Most settop terminals are tunable. A block diagram for a prior artsettop terminal is shown in FIG. 1. Incoming signals from a CATVtransmission network are coupled to an input bandpass amplifier andup-converted to a high intermediate frequency (HI-IF). The up-conversionrequires a tunable local oscillator which selects a desired channel andan associated mixer. The mixer is coupled to a bandpass filter anddown-converted to an IF channel using a fixed-frequency local oscillatorand mixer. The output channel is filtered and forwarded to asubscriber's television receiver. Prior art settop terminals use onedown-converter mixer with an oscillator having slight frequency agilityto provide an output at one or two preselected channel frequencies. Theoutput frequencies and bandwidths depend upon the transmission standardused.

In the United States, the NTSC (National Television System Committee) isthe standard for color television. Other countries have chosen differentsystems. SECAM (sequentiel couleur avec mémoire) is used by France andRussia. PAL A and PAL B (phase alternation line) are used by manyEuropean countries such as Germany and the United Kingdom. Accordingly,television receivers are typically manufactured for a specifictransmission standard. For worldwide use, a settop terminal must beadapted to the established broadcast standards.

Accordingly, there exists a need for an inexpensive method to adapt theoutput of a settop terminal to a variety of television broadcaststandards.

SUMMARY OF THE INVENTION

The present invention is a universal modulator that accepts basebandaudio and video inputs and modulated audio or data and converts thecombined signal to one of a plurality of frequencies in dependence upona desired output frequency and broadcast standard. The universalmodulator is located between baseband video and audio outputs of asettop terminal demodulator/decoder and an antenna input of a televisionreceiver or other audio/video component (such as a VCR). The universalmodulator includes a dual conversion architecture using an up-convertermixer and a down-converter mixer. Each mixer receives an oscillatorinput from a corresponding addressable, programmable, PLL (phase-lockedloop) frequency synthesizer. Each PLL frequency is controlled byfirmware in the settop terminal. Configuration is performed via manualinput using settop terminal controls, or interrogation directly by theCATV headend or by programmed settings. A communication bus coupled tothe firmware distributes addressable instructions to selectively controleach PLL frequency and obviate oscillator difference beat frequencies(ODBFs) that may be manifested.

Accordingly, it is an object of the present invention to provide auniversal modulator within a settop terminal which is able to couple aCATV transmission network to a customer's television receivernotwithstanding the broadcast standard used to transmit the televisionprograms.

Other objects and advantages will become apparent to those skilled inthis art after reading the detailed description of the preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of a preferred example, given by way of exampleand to be understood in conjunction with the accompanying drawingwherein:

FIG. 1 is a block diagram of a prior art CATV settop terminal;

FIG. 2 is a block diagram of a settop terminal incorporating the presentinvention;

FIG. 3 is a block diagram of the preferred embodiment of the universalmodulator of the present invention for use in a settop terminal;

FIG. 4 is a block diagram of an addressable, programmable, phase-lockedloop;

FIG. 5 is a flow chart of the universal modulator configuring process;

FIG. 6 is a flow chart of the ODBF translation process;

FIG. 7 is a block diagram of a prior art headend;

FIG. 8 is a block diagram of a headend made in accordance with thepresent invention; and

FIGS. 9A and 9B are graphs of oscillator difference beat frequencies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment will be described with reference to the drawingfigures where like numerals represent like elements throughout.

FIG. 2 is a block diagram of a settop terminal 17 with a universalmodulator 19 shown coupled to the outputs 23, 25, 27 of ademodulator/decoder 21. The demodulator/decoder 21 supplies a customer'schannel selection to the universal modulator 19 as a baseband audiosignal via the output 23 and as a baseband video signal via the output25. An alternate (second) audio signal, such as a NICAM carrier ormodulated audio signal which differs from the baseband audio signal, mayalso be supplied to the universal modulator 19 via the output 27 of thedemodulator/decoder 21. A reference clock signal 37 originating from amaster oscillator (not shown) and a common communication bus 39 are alsocoupled to the universal modulator 19. The functional description of thedemodulator/decoder 21 is beyond the scope of the present invention andshall not be described in further detail.

The higher quality baseband audio and video signals provided by outputs23 and 25 of the demodulator/decoder 21 are made available as settopterminal outputs 31, 33, respectively, and may be coupled to televisionreceivers that have baseband inputs. The alternate audio signal providedby output 27 may be made available as settop terminal output 29. Fortelevision receivers that lack these features, the universal modulator19 provides an up-conversion output 35 compatible with the televisionbroadcast standard used, from baseband to VHF or UHF for coupling to anantenna input.

The universal modulator 19 is shown in more detail in FIG. 3. The commoncommunication bus 39 shown is an I²C interface from Phillips®Electronics. Other bus communication protocols may alternatively beused. The configuration for a settop terminal 17 may be downloaded fromthe CATV system headend via a dedicated channel, or inband on the VBI ofa channel. One skilled in this art would appreciate that an advancedcable system can address and interrogate a specific settop terminal andalter its functionality. If the settop terminal has all configurationsstored in firmware, the CATV system headend may simply instruct thesettop terminal 17 of the standard being used. In this fashion, thesettop terminal 17 does not require a technician to configure the unitbut can auto-configure upon initial energization.

The communication bus protocol permits configuring component parametersto a particular broadcast standard using a unique addressing systemwithin the settop terminal 17. As shown in FIG. 3, the I²C bus 39communicates with: an addressable programmable PLL frequency synthesizer41 for a baseband audio mixer 69, a solid state switch 43, adjustableamplifiers 45 and 47 for the baseband video input 59 and baseband audioinput 67, an addressable programmable PLL frequency synthesizer 49 foran up-conversion mixer 91 and an addressable programmable PLL frequencysynthesizer 51 for a down-conversion mixer 101. Although the addressableprogrammable PLL frequency synthesizer 51 has been described as beingcoupled to a “down-conversion” mixer 101, the down-conversion mixer 101may in fact further up-convert a HI-IF signal 93 to a higher frequencysignal. It should be noted that each PLL frequency synthesizer 41, 49,51 has an associated oscillator driver LO1, LO2, LO3 respectively (notshown). Each respective component has its own address to permit firmwarecontained parameters to be loaded for a given broadcast standardconfiguration.

An alternate (second) audio carrier 53, provided by the output 27 of thedemodulator/decoder 21, is coupled to the solid state switch 43. Theoutput of the switch 43 is coupled to a first input 55 of a summingamplifier 57. The baseband video input 59 is coupled to a clamp 61 whichlimits signal amplitude. The output from the clamp 61 is coupled to thevideo adjustable amplifier 45 where signal gain is increased orattenuated depending upon the broadcast standard. The output from theadjustable amplifier 45 is coupled to a hard limiter 63 which clipssignal peaks. The output from the limiter 63 is coupled to a secondinput 65 of the summing amplifier 57. The baseband audio input 67 iscoupled to a baseband audio mixer 69 via an adjustable amplifier 68. Thebaseband audio mixer 69 modulates the baseband to the broadcaststandard. The baseband audio mixer 69 may be selectively activated ordeactivated by the I²C bus as required to support the standard in use.The output from the baseband audio mixer 69 is coupled to a lowpassfilter 71 to remove RF. A second input to the audio lowpass filter 71 isprovided as a modulated audio input 72. The audio lowpass filter 71 iscoupled to an audio adjustable amplifier 47 where signal gain isincreased or attenuated. The audio adjustable amplifier 47 output iscoupled to a third input 73 of the summing amplifier 57.

Each of the mixers mix a signal input with the outputs of the threeaddressable, programmable PLL frequency synthesizers 41, 49, 51. The PLLoutput frequencies vary depending on the broadcast standard and the RFoutput frequency 105 desired. An addressable, programmable PLL frequencysynthesizer 41, 49, 51 is shown in FIG. 4.

The PLL 41, 49, 51 includes a phase detector 75, a voltage-controlledoscillator (VCO) 77 and a loop filter 79. The programmable PLL usesdigital and analog techniques for frequency synthesis. The phasedetector 75 compares two input frequencies 81 a, 81 b and generates anoutput 83 that is a measure of their phase difference. If both inputs 81a, 81 b differ in frequency, the output is periodic at the differencefrequency. If the frequency input does not equal the frequency output ofthe VCO 77, the phase-error signal, after being filtered, causes the VCOfrequency to deviate in the direction of the input frequency. When thefrequencies match, the VCO 77 locks to the input frequency maintaining afixed phase relationship with the input signal. The filtered output ofthe phase detector 75 is a dc signal. A modulo-n counter 87 is coupledbetween the VCO 77 output and the second input 81 a to the phasedetector 75 to generate a multiple of the input reference frequencyproviding frequency synthesis.

Each PLL synthesizer 41, 49, 51 employed in the present invention 19 isaddressable such that the input frequency can be adjusted by using aninput modulo-n counter 89 or divide-by-n to adjust output frequency.Both the input frequency divide-by-n 89 and loop frequency divide-by-n87 are addressable components. Each of the PLLs 41, 49, 51 are addressedand controlled in accordance with a predetermined settop terminal 17configuration. The configuration determines both the output frequencyand operating bandwidth of the settop terminal 17 and adjusts the up-and down-converter PLLs 49, 51 accordingly.

Referring back to FIG. 3, the summer amplifier 57 output is modulatedwith the frequency output from the second programmable PLL 49 to drivethe up-conversion mixer 91 and translate the summed output to a highintermediate frequency (HI-IF) 93. The HI-IF 93 is higher than thehighest expected frequency in the summed amplifier 57 output bandwidth.In the present invention 19, the input to the up-conversion mixer 91 isnot bandwidth limited.

The summing amplifier 57 output frequencies are translated to a newbandwidth, starting at a low frequency of the second PLL 49 minus thehighest input band frequency, and ending at a high frequency of thethird PLL 51 minus the lowest input band frequency. The second PLL 49frequency is selected to translate the summing amplifier 57 output tocorrespond to the passband of an intermediate lowpass filter 95. Theoutput from the lowpass filter 95 is coupled to a buffer amplifier 97 torestore gain losses. The output from the buffer amplifier 97 is input toa final lowpass filter 99. The buffer amplifier 97 maintains the systemnoise figure by overcoming the losses in the up-conversion mixer 91 andfirst HI-IF filter 95. The signal is filtered by a HI-IF filter 99, withthe output coupled to a down-conversion mixer 101. The third PLLsynthesizer 51 is coupled to the down-conversion mixer 101. Thedifference between the HI-IF 93 and the third PLL 51 frequency is thedesired output channel in the IF band. It should, however, be noted thatthe down-conversion mixer 101 may accept the HI-IF 93 and furtherup-convert the signal to a higher frequency RF signal. The output isthen filtered via a low pass filter 103, (or other appropriate filter ifup-converted), and forwarded as an RF output frequency 105 for receptionby a television receiver.

As discussed above, the second 49 and third 51 programmable PLLs arecontrolled by the common communication bus 39. The bus 39 is coupled toa processor in the settop terminal demodulator/decoder 21 which receivesinstructions from the system headend or from the settop terminal's 17keypad. The configuration takes place transparently upon initialenergization of the settop terminal 17 if the system headend is equippedto send broadcast configuration instructions to the settop terminal 17.If the system headend does not have this capability, the settop terminal17 is configured via the keypad and function display (not shown). Theconfiguration request, whether from the headend or at a consumerlocation, outputs the predetermined parameters onto the I²C bus 39 foreach of the addressable components. The predetermined parameters arerelated to the standard that is being employed by the CATV system onwhich the settop terminal 17 is located. These parameters will includethe determination of whether a second audio carrier 53 exists, whetherthe baseband audio input 67 or the modulated audio input 72 are to beused and the frequency at which the RF output frequency 105 is desired.These parameters may also include any other configurable parameterswhich are employed by any of the addressable components coupled to thecommunication bus 39. It should also be recognized that since many ofthe components are addressable by the communication bus 39, a user maymanually input and address a particular component and selectivelyconfigure that component if desired.

An undesirable artifact of dual conversion is the generation ofharmonics based on the fundamental oscillator frequencies. The harmonicsof the second and third PLL frequency synthesizers 49, 51 mix with eachother, thereby creating ODBFs. To obviate the intrusive effects of thesePLL harmonics, the system and method of the present invention 19eliminate this type of interference by translating the significant ODBFsout of the desired output channel.

A flowchart of the preferred method of the present invention 19 is shownin FIG. 5. Upon making the necessary connections to the CATV cable 15and subscriber's television receiver, the settop terminal 17 isenergized (step 201) establishing communication with the system headend.If the cable system headend has forward communication ability (step205), the settop terminal is instructed how to configure itself for theapplicable broadcast standard by downloading the parameters for theregional standards being used and the channel broadcast maps (step 207).The predetermined PLL frequencies derived from the channel and broadcastmaps in memory are converted into corresponding “divide-by” numbers forthe PLL modulo-n converters 87, 89 and output to the second 49 and third51 PLL frequency synthesizers. The settop terminal 17 acknowledges whenconfiguration is complete. If the cable system does not have forwardcommunication capability, the user will be prompted to enter theapplicable information via a display and keypad, thereby manuallyloading the applicable broadcast configuration (step 209).

The settop terminal 19 reviews the loaded channel and broadcast maps.The predetermined frequencies are examined for potential ODBFs (step211). If it is determined that ODBF's are likely (step 213), an ODBFtranslation is performed (step 215) as shown in FIG. 6 (which will beexplained in greater detail hereafter). Otherwise, the originalfrequencies are maintained (step 217) (FIG. 5). The frequencies areaddressed to their respective PLL synthesizers as words over the I²Ccommunication bus (step 219).

Referring to the flow diagram of FIG. 6, the elimination of ODBFs isachieved by selectively adjusting the frequencies of the second 49 andthird PLLs 51 to obtain the desired RF output frequency. For a typicalNTSC signal, the up-converter mixer 91 modulates the input video 59 andaudio signals 67 with the output 93 of the second PLL 49 to up-convertthe input RF signal of the selected channel to the HI-IF 93 (step 301).LO1=audio carrier frequency  (Equation 1)LO2=HI-IF  (Equation 2)

The down-converter mixer mixes 101 the HI-IF 93 with the output of thethird PLL synthesizer 51 (step 303) to down-convert, (or furtherup-convert if desired), to obtain the desired RF output frequency 105.LO 3=(HI-IF)+RF output  (Equation 3)

Multiples of the second and third PLL synthesizer 49, 51 fundamentalfrequencies define the even and odd harmonics,m(LO2) and m (LO3), for m=1, 2, 3, 4, . . . ∞,  (Equation 4)which represent all possible harmonics, (step 305). However, due to thehigh system frequencies involved, examination of frequencies beyond the10th harmonic is unnecessary.

The existence of an interfering ODBF is determined by seriallycalculating the differences between two harmonics of the second 49 and51 third PLL synthesizers that are separated by at least one degreeuntil the absolute value of an ODBFm,n is within a given bandwidth or apredetermined number of ODBFm,n values are calculated. When an ODBFm,nabsolute value is found within the RF channel bandwidth, it isdesignated as an interfering oscillator difference beat frequency(ODBF). The general equation for calculating ODBFs is:ODBF _(m,n)=(m+n)(LO 2)−(m)(LO 3), form m=1, 2, 3, 4, . . .10,  (Equation 5)with n=1 for a first series, n=2 for a second series, n=3 for a thirdseries, and so on up to n=8 for all previously calculated harmonics(step 305). The ODBFm,n calculated from the differing degrees of thesecond 49 and third PLL 51 harmonics are then examined (step 307). Forexample, if the ODBF lies outside of the desired RF output channelbandwidth, no adjustment of the second 49 and third 51 PLL frequencysynthesizers is required.

For an ODBF which falls inband, the following equations can be used todetermine which direction the second 49 and third 51 PLL frequenciesshould be adjusted to translate the ODBF out of band. In theseequations, CLB is the channel low-band; CMB is the channel mid-band; andCHB is the channel high-band.If −CHB≦ODBF<−CMB; then HI-IF is moved downward.  (Equation 6A)

(If the result of Equation 5 is negative and the magnitude is greaterthan the mid-band of the desired RF output channel (step 309), the HI-IFis moved downward (step 311)).If −CMB≦ODBF≦−CLB; then HI-IF is moved upward.  (Equation 6B)

(If the result of Equation 5 is negative and the magnitude is less thanor equal to the mid-band of the desired RF output channel (step 313),the HI-IF is moved upward (step 315)).If CLB≦ODBF≦CMB; then HI-IF is moved downward.  (Equation 6C)

(If the result of Equation 5 is positive and the magnitude is less orequal to than the mid-band of the desired RF output channel (step 317),the HI-IF is moved downward (step 319)).If CMB<ODBF≦CHB; then HI-IF is moved upward.  (Equation 6D)

(If the result of Equation 5 is positive and the magnitude is greaterthan the mid-band of the desired RF output channel (step 321), the HI-IFis moved upward (step 323)).

The second 49 and third 51 PLLs are then adjusted (step 327) inaccordance with the following: To translate the oscillator differencebeats below or above the desired RF output channel, the followingequation is used to determine the Δ in frequency for the second 49 andthird 51 PLL frequency synthesizers. $\begin{matrix}{\Delta = \frac{{CMB} - \left\lbrack {{\left( {m + n} \right)({LO2})} - {m({LO3})}} \right\rbrack}{\left( {m + n} \right) - m}} & \left( {{Equation}\quad 7} \right)\end{matrix}$

The new second 49 and third 51 PLL frequencies (LO2′ and LO3′respectively) are derived as shown in LO2′ is calculated as shown inFIG. 6.

The new PLL frequencies LO2′ and LO3′ translate the ODBFs above or belowthe desired RF output channel. The new PLL frequency values are used toprogram the second 49 and third 51 PLL frequency synthesizers (step327).

In an alternative embodiment, a fixed value for Δ can be used tosimplify the calculations and the operation of the system. For example,a value of 4 MHz for Δ will suffice for NTSC and PAL systems.

The present invention will now be explained with reference to severalexamples. In the first example, if the HI-IF is 960 MHz and the desiredRF output channel has a picture carrier frequency of 319.25 MHz, we havethe following: $\begin{matrix}{{{LO2} = {{{HI}\text{-}{IF}} = {960\quad{MHz}}}};{and}} & \left( {{from}\quad{Equation}\quad 2} \right) \\\begin{matrix}{{LO3} = {{{HI}\text{-}{IF}} + {{RF}\quad{output}}}} \\{= {{960 + 319.25} = {1279.25\quad{{MHz}.}}}}\end{matrix} & \left( {{from}\quad{Equation}\quad 3} \right)\end{matrix}$

The graph for ODBFs versus the RF output frequencies for m=2 and n=1 isshown in FIG. 9A. If m=2 and n=1, then: $\begin{matrix}\begin{matrix}{{{ODBF}_{2,1}(960)} = {{\left( {m + n} \right)({LO2})} - {m({LO3})}}} \\{= {{3(960)} - {2(1279.25)}}} \\{= {{2880 - 2558.5} = {321.5\quad{MHz}}}}\end{matrix} & \left( {{from}\quad{Equation}\quad 5} \right)\end{matrix}$

Since the desired RF output channel has a picture carrier frequency of319.25 MHz (and assuming the bandwidth is 6 MHz for an NTSC channel),the ODBF is in-band for the desired RF output channel. From Equation 6D,since the ODBF is above the mid-band of the desired RF output channel,the HI-IF is moved upward. Assuming that Δ will be a fixed value of 4MHz, LO2′ will be 964 MHz and L03′ will be 1283.25 MHz. Accordingly,$\begin{matrix}\begin{matrix}{{{ODBF}_{2,1}(964)} = {{3(964)} - {2\left( {964 + 319.25} \right)}}} \\{= {{2892 - 2566.5} = {325.5\quad{{MHz}.}}}}\end{matrix} & \left( {{from}\quad{Equation}\quad 5} \right)\end{matrix}$The ODBF is now out of band.

In the second example, if the HI-IF is 960 MHz and the desired RF outputchannel has a picture carrier frequency of 481.25 MHz, we then have thefollowing: $\begin{matrix}{{{LO2} = {{{HI}\text{-}{IF}} = {960\quad{MHz}}}};{and}} & \left( {{from}\quad{Equation}\quad 2} \right) \\\begin{matrix}{{LO3} = {{{HI}\text{-}{IF}} + {{RF}\quad{output}}}} \\{= {{960 + 481.25} = {1441.25\quad{{MHz}.}}}}\end{matrix} & \left( {{from}\quad{Equation}\quad 3} \right)\end{matrix}$

The graph for ODBFs versus the RF output frequencies for m=3 and n=1 isshown in FIG. 9B. If m=3 and n=1, the ODBF can be calculated as:$\begin{matrix}\begin{matrix}{{{ODBF}_{3,1}(960)} = {{4(960)} - {3(1441.25)}}} \\{= {3840 - 4323.75}} \\{= {{- 483.75}\quad{{MHz}.}}}\end{matrix} & \left( {{from}\quad{Equation}\quad 5} \right)\end{matrix}$

Since the selected channel is 481.25 MHz, (and assuming an NTSCchannel), the ODBF is in-band and the HI-IF must be relocated. Theresult of Equation 5 for this example is negative and the magnitude isgreater than the mid-band of the desired RF output channel (481.25 MHz).Accordingly, from Equation 6A, the HI-IF is moved lower. Assuming that Δwill be a fixed value of 4 MHz, LO2′ will be 956 MHz and LO3′ will be1437.25 MHz. Recalculating the ODBF provides: $\begin{matrix}\begin{matrix}{{{ODBF}_{3,1}(956)} = {{4(956)} - {3\left( {956 + 481.25} \right)}}} \\{= {3824 - 4311.75}} \\{= {{- 487.75}\quad{{MHz}.}}}\end{matrix} & \left( {{from}\quad{Equation}\quad 5} \right)\end{matrix}$

The ODBF is now out of band.

Due to the simple design of the present invention and since there are noshielding requirements to avoid ODBFs, the universal modulator may beincorporated onto a single integrated circuit. This was not possiblewith prior art designs.

Although the present invention has been described with reference to asettop terminal, it should be understood by those of skill in the artthat the invention is adaptable to other applications within the CATVenvironment, or even other communication applications which do notpertain to CATV systems.

For example, as shown in FIG. 7, a prior art headend 700 generallyincludes two pieces of equipment; a baseband section 702 and an IFsection 704. These two sections 702, 704 are typically designed tooperate as “stand alone” units. Together, the two sections 702, 704output a single RF channel. The baseband section 702 generally comprisesa video section 706 and an audio section 708. These sections 706, 708receive audio and video baseband inputs and combine these inputs to anintermediate frequency for output to the IF section 704. In the IFsection 704, the intermediate frequency is up-converted to the desiredRF output channel. Since both sections 702, 704 comprise units ofequipment which are designed to work independently, this requires theduplication of many components between units 702, 704.

Referring to FIG. 8, a headend 800 made in accordance with the presentinvention is shown. The headend 800 includes an audio pre-processingsection 802, a video pre-processing section 804, the universal modulator808 of the present invention (which is coupled to two filters 810, 812),a transmitter 814 (if desired), and a microprocessor 806, which controlsall of the components of the headend 800. As was previously describedhereinbefore, since the universal modulator 808 can convert a basebandinput signal to any desired RF output signal while avoiding ODBFs, theuniversal modulator 808 may be utilized to replace most of thecomponents in the baseband section 702 and the IF section 704. Thissignificantly reduces the number of components required for a headend800. Accordingly, the cost and complexity are also thereby reduced.

It should be understood by those of skill in the art, with reference toFIG. 8, that the universal modulator 808 of the present invention mayalso be used to accept a baseband digital VSB signal and remodulate thesignal to a desired RF output signal for use with broadcast HDTVtelevision receivers. The universal modulator 808 could also be used totransmit RF signals to devices which require high frequency RF signals,including wireless appliances such as a cordless telephone or a wirelessLAN receiver. In such an application, the second mixer up-converts theHI-IF signal to a higher frequency RF signal, instead of down-convertingthe HI-IF as previously described. The desired RF output signal wouldbe:RF output=(HI-IF)+LO 3  (Equation 8)

The RF output signal may then be transmitted directly to the wirelessappliance.

1. A method for combining a plurality of received signals and outputtinga desired output signal, the method comprising: (a) receiving a videosignal; (b) receiving a first audio signal; (c) receiving a second audiosignal; (d) generating a first frequency; (e) mixing the first frequencywith the first audio signal to produce a modulated signal; and (f)combining the video signal, the modulated signal and the second audiosignal to generate a summed signal.
 2. The method of claim 1 furthercomprising: (g) generating a second frequency; (h) mixing the secondfrequency with the summed signal to generate a high intermediatefrequency (HI-IF) signal; (i) generating a third frequency; and (j)mixing the third frequency with the HI-IF signal to generate the desiredoutput signal.
 3. The method of claim 1 further comprising: (g) limitingthe amplitude of the video signal.
 4. The method of claim 1 furthercomprising: (g) adjusting the gain of the video signal.
 5. The method ofclaim 1 further comprising: (g) clipping signal peaks of the videosignal.
 6. The method of claim 1 further comprising: (g) adjusting thegain of the first audio signal.
 7. A modulator for combining a pluralityof received signals and outputting a desired output signal, themodulator comprising: (a) a first frequency synthesizer for generating afirst frequency; (b) an audio mixer in communication with the firstfrequency synthesizer, the audio mixer for mixing an audio signal withthe first frequency to generate a mixed audio signal; (c) a summingamplifier having a first input for receiving a video signal, a secondinput for receiving the mixed audio signal, and an output for outputtinga modulated summed signal; (d) a second frequency synthesizer forgenerating a second frequency for mixing with the modulated summedsignal to generate a high intermediate frequency (HI-IF) signal; and (e)a third frequency synthesizer for generating a third frequency formixing with the HI-IF signal to generate the desired output signal. 8.The modulator of claim 7 wherein the audio mixer is configured to beselectively activated or deactivated.
 9. The modulator of claim 7further comprising: (f) an up-conversion mixer having an inputelectrically coupled to the output of the summing amplifier forreceiving the modulated summed signal and outputting the HI-IF signal.10. The modulator of claim 9 wherein the up-conversion mixer iselectrically coupled to the second synthesizer for receiving the firstfrequency for mixing with the modulated summed signal to generate theHI-IF signal.
 11. The modulator of claim 7 further comprising: (f) adown-conversion mixer for receiving the HI-IF signal and outputting thedesired output signal.
 12. The modulator of claim 11 wherein thedown-conversion mixer is electrically coupled to the third frequencysynthesizer for receiving the third frequency for mixing with the HI-IFsignal to generate the desired output signal.
 13. The modulator of claim7 further comprising a common communication bus, electrically coupled tothe first, second and third synthesizers, for programming the first,second and third frequencies.
 14. The modulator of claim 7 furthercomprising a clamp for limiting the amplitude of the video signal. 15.The modulator of claim 14 further comprising an adjustable amplifier,electrically coupled to the clamp, for adjusting the gain of the videosignal.
 16. The modulator of claim 15 further comprising a commoncommunication bus, electrically coupled to the adjustable amplifier, forcontrolling the gain adjustment.
 17. The modulator of claim 15 furthercomprising a limiter, electrically coupled to the adjustable amplifier,for clipping signal peaks of the video signal.
 18. The modulator ofclaim 12 further comprising an adjustable amplifier for adjusting thegain of the audio signal.
 19. The modulator of claim 18 furthercomprising a common communication bus, electrically coupled to theadjustable amplifier, for controlling the gain adjustment.
 20. Themodulator of claim 7 wherein the modulator is incorporated into a cabletelevision (CATV) settop box.
 21. The modulator of claim 20 wherein thedesired output signal is coupled to a television receiver external tothe modulator.
 22. A method for combining a plurality of receivedsignals and outputting a desired output signal, the method comprising:(a) receiving a video signal; (b) receiving an baseband audio signal;(c) generating a first frequency; (d) mixing the first frequency withthe baseband audio signal to produce a broadcast standard signal; (e)combining the video signal and the broadcast standard signal to generatea modulated summed signal; (f) generating a second frequency; (g) mixingthe second frequency with the modulated summed signal to generate a highintermediate frequency (HI-IF) signal; and (h) generating a thirdfrequency; and (i) mixing the third frequency with the HI-IF signal togenerate the desired output signal.
 23. A method for combining aplurality of received signals and outputting a desired output signal,the method comprising: (a) receiving a video signal; (b) processing thevideo signal to produce a processed video signal by: (b1) limiting theamplitude of the video signal, (b2) adjusting the amplitude of the videosignal, and (b3) clipping signal peaks of the video signal; (c)receiving a baseband audio signal; (d) processing the baseband audiosignal by: (d1) adjusting the amplitude of the baseband audio signal,(d2) generating a first frequency, (d3) mixing the first frequency withthe baseband audio signal to produce a broadcast standard signal, (d4)lowpass filtering the broadcast standard signal, and (d5) adjusting theamplitude of the broadcast standard signal; and (e) combining theprocessed video signal and the broadcast standard signal to generate amodulated summed signal.
 24. The method of claim 23 further comprising:(f) generating a second frequency; (g) mixing the second frequency withthe modulated summed signal to generate a high intermediate frequency(HI-IF) signal; (h) generating a third frequency; and (i) mixing thethird frequency with the HI-IF signal to generate the desired outputsignal.
 25. A modulator for combining a plurality of received signalsand outputting a desired output signal, the modulator comprising: (a) afirst input for receiving a video signal; (b) a second input forreceiving an baseband audio signal; (c) means for generating a firstfrequency; (d) means for mixing the first frequency with the basebandaudio signal to produce a broadcast standard signal; (e) means forcombining the video signal and the broadcast standard signal to generatea modulated summed signal; (f) means for generating a second frequency;(g) means for mixing the second frequency with the modulated summedsignal to generate a high intermediate frequency (HI-IF) signal; (h)means for generating a third frequency; and (i) means for mixing thethird frequency with the HI-IF signal to generate the desired outputsignal.
 26. A modulator for combining a plurality of received signalsand outputting a desired output signal, the modulator comprising: (a) avideo signal processing circuit including: (a1) a clamp for receivingand limiting the amplitude of a video signal, (a2) a first adjustableamplifier for receiving the video signal from the clamp and adjustingthe amplitude of the video signal, and (a3) a limiter for receiving thevideo signal from the first adjustable amplifier and clipping signalpeaks of the video signal; (b) a baseband audio signal processingcircuit including: (b1) a second adjustable amplifier for receiving andadjusting the amplitude of a baseband audio signal, (b2) a phase-lockedloop (PLL) synthesizer for generating a first frequency, (b3) a basebandaudio mixer for receiving the baseband audio signal from the secondadjustable amplifier and mixing the first frequency with the basebandaudio signal to produce a broadcast standard signal, (b4) a lowpassfilter for receiving the broadcast signal from the baseband audio mixerand filtering the broadcast standard signal, and (b5) a third adjustableamplifier for receiving the broadcast standard signal from the lowpasssignal and adjusting the amplitude of the broadcast standard signal; and(c) a summing amplifier having a first input electrically coupled to anoutput of the third adjustable amplifier, and a second inputelectrically coupled to an output of the limiter, the summing amplifierfor combining the processed video signal and the broadcast standardsignal to generate a modulated summed signal.
 27. The modulator of claim26 further comprising: (d) a switch for selectively inputting a secondaudio signal to a third input to the summing amplifier, wherein thesumming generator combines the processed video signal, the broadcaststandard signal and the second audio signal to generate the modulatedsummed signal.
 28. The modulator of claim 27 further comprising: (e) acommunication bus in communication with the switch, the first adjustableamplifier, the second adjustable amplifier, the third adjustableamplifier, the PLL synthesizer and the baseband audio mixer.